The present invention relates to a method and apparatus for providing wireline depth measurements for a bore-hole tool lowered in the bore hole on a wireline such as during well-logging operations.
One method in common use for providing a wireline depth measurement involves the use of encoder wheels which frictionally engage the cable or wireline to detect its movement. One such system is described in U.S. Pat. No. 4,179,8I7, the disclosure of which is hereby incorporated by reference into the present specification. In this type of system, two encoder wheels are used for redundancy. As the cable moves, the wheels turn and, by counting the revolutions of the wheel, the amount cf cable going into the hole can be measured. As the wheels turn, pulses are produced which are then counted by appropriate electronic circuitry. Since the wheels are not perfectly round and may be of different sizes, a wheel correction may also be applied to the measurement. This correction eliminates systematic errors due to physical differences in the wheels. For example, if one wheel is larger than the other so that it produces one less pulse every 10,000 pulses than the smaller wheel, a wheel correction is applied by counting every 10,000th pulse from the larger wheel twice.
Other sources of error during this type of depth measurement are not systematic, however. That is, they occur randomly. A major source of such random error is slippage of the encoder wheel. If the surface of the cable is slipping on the surface of the encoder wheel, some of the motion of the cable will go undetected. Prior methods have attempted to correct for this by simply averaging the depths measured by each wheel. The rational for this is that errors in the measurements from each wheel will cancel out. However, slippage errors are almost invariably unidirectional and both measuring wheels rarely slip simultaneously. This means that if only one of the two encoder wheels is slipping, averaging the two together is not the optimum thing to do. In fact, with averaging, the depth from the correct wheel is corrupted by the depth from the wheel that has slipped. The result is a measurement that is not as bad as the depth measured by the wheel which has slipped but not as good as the depth from the wheel which has not slipped.
For example, in the system described in U.S. Pat. No. 4,179,817, independent signals corresponding to the rotation of each encoder wheel are compared to determine which signal corresponds to the more rapidly rotating wheel. Where the comparison finds the signals differ only by a small percentage, the signals are automatically averaged to provide the depth indication. Where the difference between the signals is more than a small percentage, the signal from the apparently less rapidly rotating wheel is disregarded. The system provides for indicating the apparently slipping wheel to alert the operator so that appropriate action may be taken. Thus, in that system, the measurement for: the non-slipping wheel is corrupted by averaging it with the measurement from the slipping wheel except in those cases where the wheel slips so much that it is disregarded thereafter.
It is, therefore, an object of the present invention to provide a method for measuring wireline depths which corrects for encoder wheel slippage in a manner which does not corrupt the measurement from a non-slipping encoder wheel.